High efficiency traveling wave tube employing harmonic bunching

ABSTRACT

A slow wave tube is disclosed wherein the signal to be amplified and an harmonic thereof are applied concurrently over at least a portion of the slow wave circuit to increase the RF conversion efficiency of the tube. In a preferred embodiment, the harmonic wave energy is extracted from the output of the tube and fed back onto the slow wave circuit. The slow wave circuit portion, which provides the harmonic interaction with the beam, preferably has a dispersive characteristic such that the harmonic wave energy has a higher phase velocity than the fundamental wave energy for enhanced RF conversion efficiency.

United States Patent Lien June 6, 1972 54] HIGH EFFICIENCY TRAVELINGWAVE 3,243,735 3/1966 Gross ..315/3.5 x TUBE EIMPLOYING HARMONIC3,335,314 8/1967 Espinosa et a1.. ...315/3.5 X CHI 3,366,897 1/1968Konrad ..315/3.5 X BUN NG 3,448,330 6/1969 Farney ..3l5/3.6 [72]Inventor: Erling L. Lien, Los Altos, Calif.

Primary Examiner-Herman Karl Saalbach [73] Asslgnee' vanan AssoclatesPalo Alto Cahf' Assistant Examiner-Saxfield Chatmon, Jr. [22] Filed;Sept 3, 7 Attorney-Stanley Z. Cole 1 1 pp 69,295 57 ABSTRACT A slow wavetube is disclosed wherein the signal to be am-' [52] U.S. Cl ..330/43,315/35 plified and an harmonic thereof are applied concurrently over[51] Int. Cl. ..H0lj 25/34 at least a portion of the slow wave circuitto increase the RF [58] Field of Search ..315/3.5 X, 3.6, 393; 330/43nversion fficien y f h tube- I a preferred im n the harmonic wave energyis extracted from the output of the 5 R f r Cited tube and fed back ontothe slow wave circuit. The slow wave circuit portion, which provides theharmonic interaction with UNITED STATES PATENTS the beam, preferably hasa dispersive characteristic such that the harmonic wave energy has ahigher phase velocity than the 3,128,433 4/1964 Edson ..315/3-.5 Xfundamental wave energy for enhanced RF conversion effb 2,829,252 4/1958Bryant ..315/3.5 X ciency 2,681,951 6/1954 Warnecke et al .....315/3.5 X2,811,664 10/1957 Kazan ..315/3.6 9Claims, 4Drawing Figures FATENTEDJUH61972 3, 668 544 L L K 24 ELECTRON ELECTRON ELECTRON M 22 BUNCH 22 BUNCH22 BUNCH INVENTOR.

ERLING L. LIEN Sw-EFW ATTORNEY HIGH EFFICIENCY TRAVELING WAVE TUBEEMPLOYING HARMONIC BUNCI-IING DESCRIPTION OF THE PRIOR ART Heretofore,it has been proposed that the efiiciency of a slow wave tube could beenhanced at the fundamental frequency by driving the slow wave circuitwith the second harmonic of the output signal having a suitably adjustedamplitude and phase for counteracting undesired harmonic interaction inthe tube. In the prior art, the coherent harmonic signal for driving theslow wave circuit was obtained from the output of a driver tube in apreceding stage which also served as a preamplifier for the tube whichwas to have the improved RF efficiency. Thus, it is known from the priorart to apply second harmonic energy simultaneously with the fundamentalenergy to be amplified onto the slow wave circuit of a traveling wavetube to counteract the normal second harmonic interaction in the beam toimprove the efiiciency, particularly at the low frequency end of abroadband traveling wave tube. Such a broadband tube is described in anarticle titled, "Ultra- Broadband TWT Power Amplifiers Without HarmonicCapture," appearing in a paper delivered at the lntemational ElectronDevice meeting in Washington, D. C. in October, 1965.

It is also known from the prior art that the conversion efficiency of aklystron amplifier tube can be substantially increased by prebunchingthe beam with second harmonic ener gy. Such an improved klystronamplifier is disclosed and claimed in copending U. S. Pat. applicationsSer. No. 767,774 filed Oct. 15, I968; Ser. No. 20,791 filed Apr. 15,1970; and Ser. No. 28,792 filed Apr. 15, 1970, all assigned to the sameassignee as the present invention.

- SUMMARY OF THE INVENTION The principal object of the present inventionis the provision of an improved traveling wave tube amplifier havingincreased RF conversion efficiency.

One feature of the present invention is the provision, in a travelingwave tube amplifier, of harmonic feedback means coupled to the output ofthe slow wave circuit for coupling harmonic energy from the output ofthe tube back on to the slow wave circuit for additional electromagneticinteraction with the beam to increase the RF conversion efficiency ofthe tube.

Another feature of the present invention is the provision, in atraveling wave tube, of means for coupling onto the slow wave circuitharmonic wave energy for harmonic interaction with beam, such slow wavecircuit having a dispersive characteristic such that the phase velocityof the harmonic wave energy exceeds the phase velocity of thefundamental wave energy, whereby the conversion efiiciency of the tubeis increased at the fundamental frequency.

Another feature of the present invention is the same as any one or moreof the preceding features including the provision of an input circuitdisposed upstream of the second harmonic interaction circuit, such inputslow wave circuit portion being terminated in a resistive load such asto be severed from the second harmonic interaction circuit disposeddownstream of the input circuit.

Another feature of the present invention is the same as any one or moreof the preceding features wherein the second harmonic interaction slowwave circuit includes a conductive sheath surrounding the interactioncircuit and capacitive loading means, such as conductive vanes,extending from the sheath towards the slow wave circuit for producing adispersive characteristic wherein the harmonic wave energy travels at afaster phase velocity than the fundamental wave energy.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic line diagram depictinga traveling wave tube incorporating features of the present invention,

FIG. 2 is a plot of axial electric field for the fundamental and secondharmonic waves as a function of distance along the beam path anddepicting the relative positions of the electron bunches and theelectric fields for three different regions within the second harmonicinteraction region of the tube of FIG. 1,

FIG. 3 is a dispersion diagram depicting the dispersive characteristicsof the second harmonic interaction circuit of FIG. 1, and

FIG. 4 is a transverse sectional view of the structure of FIG. 1 takenalong line 4-4 in the direction of the arrows and depicting, inschematic line diagram form, the slow wave circuit and surroundingcaPacitively loaded sheath.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thereis shown a traveling wave tube 1 incorporating features of the presentinvention. The traveling wave tube 1 includes an electron gun assembly 2disposed at one end of the tube for formingv and projecting a beam ofelectrons 3 over an elongated beam path to a beam collector structure 4.

An input slow wave circuit portion 5 is disposed near the upstream endof the beam 3. The input slow wave circuit may comprise any one of anumber of different kinds of slow wave circuits, such as a helix, across-wound helix, a ring and bar circuit, a succession of floatingbuncher cavities, a coupled cavity circuit, a cloverleaf circuit, or thelike. The input end of the slow wave circuit 5 includes RF couplingmeans 6 for coupling wave energy to be amplified at a fundamentalfrequency onto the input of the slow wave circuit 5. The downstream endof the slow wave circuit 5 is terminated with a refiectionless waveattenuative element 7 such as a resistive card. Termination 7 serves asa circuit sever for severing the input circuit 5 from the remainingportion of the slow wave circuit of the tube.

A second harmonic interaction slow wave circuit 8 is disposed downstreamfrom the input circuit 5. The second harmonic interaction circuit 8 maycomprise any one of a number of different types of slow wave circuits,such as a helix, a cross-wound helix, a bifilar helix, a ring and barcircuit, a cloverleaf circuit, a coupled cavity circuit, etc. An outputcoupler 9 is disposed at the downstream end of the second harmoniccircuit 8 for coupling output energy from slow wave circuit 8 to autilization device, such as an antenna, not shown.

A power splitter l 1 is provided at the end of the output circuit 8,near the output coupler 9, for selectively coupling out of the outputwave energy the energy which is harmonically related to the fundamentalenergy being amplified on the circuit. A feedback circuit, including aseries connection of a variable attenuator l3 and a variable phaseshifter 14, feeds the harmonic wave energy back to the input or upstreamend of the second harmonic interaction circuit 8.

The second interaction circuit 8 is dimensioned for cumulativeinteraction with the beam at the fundamental frequency of wave energy tobe amplified. The circuit is also capable of interacting with the beamat the second harmonic or at higher harmonics of the fundamentalfrequency. In a preferred embodiment, the second harmonic interactioncircuit 8 has a negative dispersion characteristic as shown by the solidcurve 15 of FIG. 3, such that the phase velocity of the second harmonicand higher harmonics is substantially higher than the phase velocity forthe fundamental wave on the circuit 8. In the typical example of a helixslow wave circuit, the normal dispersive characteristic for the helix asenclosed in a sheath is as depicted by the dashed line 16 of FIG. 3,such curve 16 having a positive dispersive characteristic. The positivedispersive characteristic 16 is converted to a negative dispersivecharacteristic as depicted by curve 15 by providing capacitive loadingof the helix to the sheath.

Referring now to FIG. 4, there is shown a capacitively loaded helix slowwave circuit for producing the negative dispersive characteristic, asdepicted by curve 15 of FIG. 3.

More specifically, the helix 8 is surrounded by conductive sheath 19, asof copper, to which a plurality of conductive fins 21 are affixed andwhich project from the surrounding sheath 19 toward the helix 8 toprovide capacitive loading between the sheath and the helix. Theconductive fins 21 extend longitudinally of the sheath as well asprojecting inwardly therefrom.

Referring now to FIG. 2, the bunching action produced by the secondharmonic interaction with the electron beam is depicted. Morespecifically, at the upstream end of the second harmonic interactioncircuit 8, in the region identified as A, the electron bunches arephased relative to the fundamental electric field of the fundamentalwave on the circuit such that the electron bunches are locatedsubstantially at the node of the fundamental electric field, as depictedin that portion of FIG. 2 identified as A. The second harmonic wavewhich is fed onto the interaction circuit 8 is preferably phased, in theupstream region A, such that the second harmonic is in phase with thefundamental. This produces a composite wave as shown by the sawtoothwave 24. This improves the effectiveness of the bunching by removingundesired electrons from the interbunch region.

Due to the dispersive characteristics of the circuit 8, the secondharmonic wave advances in phase relative to the phase of the fundamentaland the electron bunches move slightly ahead of the fundamental as shownin the region identified by region B of FIG. 2. Region B corresponds tothat portion of the circuit identified as B in FIG. 1. In region B, theelectron bunches are delivering energy to both the fundamental andsecond harmonic components of the electric field on the circuit. Thefundamental component of energy delivered from the beam to the circuitcreates the growing fundamental wave on the circuit 8 and the energydelivered at the second harmonic from the electron bunches to thecircuit provides a second harmonic wave which will be extracted from thecircuit to provide the second harmonic drive in region A.

Referring now to that portion of FIG. 2 identified by C, whichcorresponds to the region of circuit 8 identified by c, in this regionthe electron bunch continues to deliver energy to the fundamental wave22, thereby further contributing to the growing fundamental wave on thecircuit. In addition, the second harmonic wave 23 has new advanced to aposition tending to rebunch the electrons into the electron bunch tocompensate for space charge forces tending to spread the electron bunch.

Thus, it is seen that the second harmonic energy which is fed back ontothe second harmonic interaction circuit 8 substantially improves theelectron bunching and significantly contributes to increased RFconversion efi'iciency.

As an alternative use of the severed circuit, the slow wave circuit maybe continuous with the feedback being applied at the input end of thetube. In addition, it is possible to derive the coherent second harmonicor higher harmonic wave from the preamplifier or driver tube and to feedthe second harmonic wave energy onto the circuit, at the upstream end ofcircuit 8 or at the upstream end of the slow wave circuit.

The phase shifter 14 and attenuator 13 are adjusted to obtain the properphasing of the harmonic wave energy with the fundamental to obtainoptimum RF conversion efficiency.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a slow wave tube, means for projecting a beam of electrons over anelongated beam path, slow wave circuit means disposed along the beampath for electromagnetic interaction with the beam to produce outputwave energy of a fundamental frequency and of harmonics of thefundamental frequency, said harmonics being integral multiples of saidfundamental frequency, output coupling means for extracting the outputwave energy from the tube, the improvement comprising,

harmonic feedback means coupled to said output coupling means forcoupling out of the output wave energy at least a portion of theharmonic energy and for feeding the harmonic wave energy back onto saidslow wave circuit means at a feedback point upstream of said outputcoupling means for electromagnetic interaction with the beam to increasethe RF conversion efficiency of the tube at the fundamental frequency.2. The apparatus of claim 1 wherein said harmonic feedback meansincludes, variable phase shifting means for variably shifting the phaseof the harmonic energy fed back to said circuit means.

3. The apparatus of claim 1 wherein said harmonic feedback meansincludes, variable attenuator means for attenuating the harmonic energyfed back to said circuit means.

4. The apparatus of claim 1 wherein said slow wave circuit meansincludes a portion disposed intermediate said feedback point and saidoutput coupling means which has a dispersive characteristic wherein thephase velocity of the harmonic wave energy substantially exceeds .thephase velocity of the fundamental wave energy for enhancing the RFconversion efficiency of the tube at the fundamental frequency.

5. The apparatus of claim 4 wherein said slow wave circuit portiondisposed intermediate said feedback point and said output coupling meanscomprises, conductive sheath means surrounding said slow wave circuitmeans in spaced relation therefrom, and capacitive loading meansextending from said surrounding conductive sheath means toward said slowwave circuit for producing the aforementioned dispersive characteristicof said slow wave circuit portion.

6. The apparatus of claim 1 wherein said slow wave circuit meansincludes an input slow wave circuit portion disposed along the beam pathupstream of said feedback point, and means for applying wave energy atthe fundamental frequency to said input circuit portion.

7. The apparatus of claim 6 including, circuit sever means at thedownstream end of said input circuit means for terminating said inputslow wave circuit portion.

8. In a slow wave tube, means for projecting a beam of electrons over anelongated beam path, slow wave circuit means disposed along the beampath for electromagnetic interaction with the beam to produce outputwave energy of a fundamental frequency and of harmonic frequencies ofthe fundamental frequency, output coupling means for extracting theoutput wave energy of the tube, the improvement comprising,

means for applying wave energy which is an integral multiple of thefundamental frequency to said slow wave circuit means over a harmonicinteraction region for harmonic electromagnetic interaction with thebeam, and

wherein said harmonic interaction region of said slow wave circuit has adispersive characteristic such that the phase velocity of the harmonicwave energy exceeds the phase velocity of the fundamental wave energy toincrease the RF conversion energy of the tube at the fundamentalfrequency.

9. The apparatus of claim 8 including, conductive sheath meanssurrounding said slow wave circuit means in spaced relation therefrom,and capacitive loading means extending from said surrounding conductivesheath means toward said slow wave circuit for producing theaforementioned dispersive characteristic of said slow wave circuitportion.

1. In a slow wave tube, means for projecting a beam of electrons over anelongated beam path, slow wave circuit means disposed along the beampath for electromagnetic interaction with the beam to produce outputwave energy of a fundamental frequency and of harmonics of thefundamental frequency, said harmonics being integral multiples of saidfundamental frequency, output coupling means for extracting the outputwave energy from the tube, the improvement comprising, harmonic feedbackmeans coupled to said output coupling means for coupling out of theoutput wave energy at least a portion of the harmonic energy and forfeeding the harmonic wave energy back onto said slow wave circuit meansat a feedback point upstream of said output coupling means forelectromagnetic interaction with the beam to increase the RF conversionefficiency of the tube at the fundamental frequency.
 2. The apparatus ofclaim 1 wherein said harmonic feedback means includes, variable phaseshifting means for variably shifting the phase of the harmonic energyfed back to said circuit means.
 3. The apparatus of claim 1 wherein saidharmonic feedback means includes, variable attenuator means forattenuating the harmonic energy fed back to said circuit means.
 4. Theapparatus of claim 1 wherein said slow wave circuit means includes aportion disposed intermediate said feedback point and said outputcoupling means which has a dispersive characteristic wherein the phasevelocity of the harmonic wave energy substantially exceeds the phasevelocity of the fundamental wave energy for enhancing the RF conversionefficiency of the tube at the fundamental frequency.
 5. The apparatus ofclaim 4 wherein said slow wave circuit portion disposed intermediatesaid feedback point and said output coupling means comprises, conductivesheath means surrounding said slow wave circuit means in spaced relationtherefrom, and capacitive loading means extending from said surroundingconductive sheath means toward said slow wave circuit for producing theaforementioned dispersive characteristic of said slow wave circuitportion.
 6. The apparatus of claim 1 wherein said slow wave circuitmeans includes an input slow wave circuit portion disposed along thebeam path upstream of said feedback point, and means for applying waveenergy at the fundamental frequency to said input circuit portion. 7.The apparatus of claim 6 including, circuit sever means at thedownstream end of said input circuit means for terminating said inputslow wave circuit portion.
 8. In a slow wave tube, means for projectinga beam of electrons over an elongated beam path, slow wave circuit meansdisposed along the beam path for electromagnetic interaction with thebeam to produce output wave energy of a fundamental frequency and ofharmoniC frequencies of the fundamental frequency, output coupling meansfor extracting the output wave energy of the tube, the improvementcomprising, means for applying wave energy which is an integral multipleof the fundamental frequency to said slow wave circuit means over aharmonic interaction region for harmonic electromagnetic interactionwith the beam, and wherein said harmonic interaction region of said slowwave circuit has a dispersive characteristic such that the phasevelocity of the harmonic wave energy exceeds the phase velocity of thefundamental wave energy to increase the RF conversion energy of the tubeat the fundamental frequency.
 9. The apparatus of claim 8 including,conductive sheath means surrounding said slow wave circuit means inspaced relation therefrom, and capacitive loading means extending fromsaid surrounding conductive sheath means toward said slow wave circuitfor producing the aforementioned dispersive characteristic of said slowwave circuit portion.